Ascorbic acid vitamin stability

Interaction of ascorbic acid (vitamin C) with the silica surface was complicated by the fast oxidation of ascorbic acid in aqueous and ethanol solutions because of dissolved oxygen.12,13 However, both unmodified and modified silica increase the oxidation resistance of vitamin C. In particular, the rate of ascorbic acid oxidation to dehydroascorbic acid was found to be much less in the presence of unmodified or modified silica (Figure 4). Vitamin C is stabilized in the presence of silica, apparently due to interaction of the vitamin with the surface of highly-disperse silica particles, as confirmed by the results... 

It has been found that glucose, sucrose, casein, albumin, sodium chloride, flour groats, and starch stabilize ascorbic acid (vitamin C).771 The addition of 5% of starch inhibits the decomposition of ascorbic acid.772 Studies on the stability of ascorbic acid, sodium ascorbate, erythorbic acid, and sodium erythorbate showed that some stabilization is observed after the addition of 8% starch.773 It was reported that the interaction is purely physical in nature.774... 

Ascorbic acid, vitamin C, has one very acidic H (p fal =4.1), and all the others are not very acidic (p Ta2 =11.8). Use your knowledge of anion stability and resonance to find the acidic H and explain why it is so acidic. Draw all the resonance forms of the conjugate base. 

Product stability and performance can be affected by exposure to several oxidative sources, including oxygen, free radicals, UV radiation, oxidative enzymes, catabolic oxidation, and chemical oxidation. Many antioxidants are also good UV absorbers due to their conjugated chemical structure. Typical antioxidants found in cosmetic products are flavonoids, polyphenols, carotenoids, thiols, tocopherol (vitamin E) and ascorbic acid (vitamin C) [71,72], According to Black [73], a combination of antioxidants from different classes is more effective than a single antioxidant due to an antioxidant cascade mechanism. 

Parenteral formulations often contain excipients considered to be chemically stable and inert however, all excipients in a formulation may influence the photochemical stability of the product. Dextrose and sodium chloride are used to adjust tonicity in the majority of parenteral formulations. Sodium chloride can affect photochemical processes by influencing solvation of the photoreactive molecules (see Section 14.2.3). The ionic strength is reported to affect the photochemical decomposition rate of minoxidil until a saturation level is reached (Chinnian and Asker, 1996). The photostability of L-ascorbic acid (vitamin C) in aqueous solution is enhanced in the presence of dextrose, probably caused by the scavenging effect of the excipient on hydroxyl radicals mediated by the photolysis of ascorbic acid sucrose, sorbitol, and mannitol have the same effect (Ho et al., 1994). Monosaccharides (dextrose, glucose, maltose, and lactose), disaccharides (sucrose and trehalose), and polyhydric alcohols (inositol, mannitol, and sorbitol) are examples of commonly used lyo-additives in parenterals. These excipients may also affect photochemical stability of the products after reconstitution. 

Ascorbic acid (vitamin C) is utilized as a cofactor to stabilize the chloroplast stroma, in quenching free radicals and reacting with hydroxy radicals and in the biosynthesis of tartaric acid and oxalic acid (6), important organic acids in grapes, and many vegetables. The effect of CA on ascorbic acid content differs with commodity,... 

Ascorbic acid (Vitamin C, cf. Figure 4.4) exists in cells, often in rather high (millimolar) concentrations, as a negatively charged lactone that is very susceptible to one-electron oxidation (Swartz and Dodd, 1981), giving a free radical that is rather stable due to resonance stabilization. Further one-electron oxidation affords... 

Ascorbic acid (vitamin C) possesses a high antioxidant activity and can be readily detected at low potentials at GCEs. Iwase [99, 114] has studied its occurrence in several foods and beverages by isocratic elution using mobile phases containing amino acids and nucleic acids instead of inorganic salts to increase the stability and reproducibility of such assays. Similar examples using conventional mobile phases have been reported [100-102]. 

Although there are several compounds (Ascorbic acid (vitamin C) or alpha-tocopherol (vitamin E) which their antioxidant effect is proved to be effective in reducing the oxidation of UHMWPE [60], there are several studies that support the antioxidant capacity of carbon nanotubes (CNTs). For example, in the study of Zeynalov et al. [61] was conducted a simulation of thermo-oxidative processes which take place in the polymer chains, and the results showed an inhibition of oxidation of the polymer when carbon nanotubes are present. Also, P. Castell et al. [62] founded that the incorporation of low concentration of arc-discharged multiwall carbon nanotubes (MWCNTs) can act as inhibitors of the oxidative process on irradiated UHMWPE, proving the radical scavenger effect of this reinforcing material. Also, this study shows that the presence of MWCNTs enhances the chemical stability of the polymer. 

Physical Properties. Table 3 contains a summary of the physical properties of L-ascorbic acid. Properties relating to the stmcture of vitamin C have been reviewed and summarized (32). Stabilization of the molecule is a consequence of delocalization of the TT-electrons over the conjugated enediol system. The highly acidic nature of the H-atom on C-3 has been confirmed by neutron diffraction studies (23). 

Stability. Ascorbic acid, a white crystalline compound, is very soluble ia water and has a sharp, acidic taste. In solution, the vitamin oxidizes on exposure to air, light, and elevated temperatures. Solutions of ascorbic acid turn yellowish, followed by development of a tan color. Ascorbic acid is stable to air when dry but gradually darkens on exposure to light. 

The selection of an appropriate antioxidant depends on factors such as stability, toxicity, efficiency, odor, taste, compatibility with other ingredients, and distribution phenomena between the two phases. Antioxidants that give protection primarily in the aqueous phase include sodium metabisulfite, ascorbic acid, thioglycerol, and cysteine hydrochloride. Oil-soluble antioxidants include lecithin, propyl gal-late, ascorbyl palmitate, and butylated hydroxytoluene. Vitamin E has also been used, but its virtue as a natural antioxidant has been the subject of some controversy. 

Choy et al. [193] reported that vitamin A (retinoic acid), vitamin C (ascorbic acid) and vitamin E (tocopherol) could be intercalated into Zn/Al LDHs by the coprecipitation method. In solutions, these vitamins are normally all sensitive to light, heat and oxygen, and it was proposed that incorporating the molecules into a layered inorganic lattice may lead to their stabilization, resulting in a wider range of potential apphcations. 

Vitamin C (ascorbic acid) is also a well-known antioxidant. It can readily lose a hydrogen atom from one of its enolic hydroxyls, leading to a resonance-stabilized radical. Vitamin C is acidic (hence ascorbic acid) because loss of a proton from the same hydroxyl leads to a resonance-stabilized anion (see Box 12.8). However, it appears that vitamin C does not act as an antioxidant in quite the same way as the other compounds mentioned above. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

The stability of some vitamins is influenced by aw. In general, the stability of retinol (vitamin A), thiamin (vitamin Bj) and riboflavin (vitamin B2) decreases with increasing aw. At low av (below 0.40), metal ions do not have a catalytic effect on the destruction of ascorbic acid. The rate of loss of ascorbic acid increases exponentially as aw increases. The photodegradation of riboflavin (Chapter 6) is also accelerated by increasing aw. 

Ascorbic acid, known more familiarly as vitamin C, is used not only as on acidulant but also as a stabiliser within the soft drinks system, and its antioxidant properties serve to improve the shelf-fife stability of flavour components. Many of the ingredients used in flavourings are susceptible to oxidation, particularly aldehydes, ketones and keto-esters. Ascorbic acid shields these from attack by being preferentially oxidised and lost, leaving the flavour component unaffected. 

In connection with the antioxidant properties of L-ascorbic acid and its stability, many kinetic and mechanistic studies have been performed. For instance, it has been shown a role as a radical scavenger in the autooxidation of methyl linoleate, and its synergistic effect when used with vitamin E.362 The photooxidation,363 superoxide-mediated oxidations,364 reactions with radicals,365 and the influence of other agents, including ultrasound and y-rays,366 have been reported. 

One of the main problems of topical application of vitamin C is that it is extremly unstable, so hydrophilic derivatives like sodium ascorbyl phosphate and lipophilic esters with fatty acids were synthesized to improve stability.43,44 However, an efficient increase in vitamin C levels after topical application of different ascorbic acid derivatives including magnesium ascorbyl phosphate, ascorbyl-6-palmitate, and dehydroascorbic acid to porcine skin could not be shown.42... 

M3. Marcus, M., Prabhudesai, M., and Wassef, S., Stability of vitamin B12 in the presence of ascorbic acid in food and serum restoration by cyanide of apparent loss. Am. ]. Clin. Nutr. 33, 137-143 (1980). 

---